Papers by Keyword: Austenite

Abstract: Owing to its superior corrosion resistance properties, thin sheet austenitic stainless-steel grade 304 is widely used in the food and beverage industry. Resistance spot welding (RSW) is a preferred joining method for these sheets because of its speed and efficiency, but RSW can alter the microstructure of the weldments, influencing their strength and corrosion resistance. Parameters such as welding current, time, and electrode force influence weld nugget formation, affecting its shape, size, and strength, while the rapid cooling rate in RSW leads to the formation of skeletal, lathy, and acicular delta ferrite in the weld zone. This study investigates the effect of welding parameters on the microstructure and properties of 0.4 mm AISI 304 stainless steel using peel tests, microstructural analysis, and microhardness testing. Peel test results were used to construct the weld lobe curve for process optimization, and nugget formation under various welding conditions was examined using SEM and EDAX analysis. The pseudo binary phase diagram was used to predict the final weld microstructure, and microhardness measurements confirmed an increase in fusion zone hardness after resistance spot welding. The weld lobe curve revealed that optimal defect free welds without expulsion were obtained at 40 kgf with 3000 to 5000 amperes for 1 to 4 cycles, and at 30 kgf with 5000 to 6000 amperes for 3 to 6 cycles, with electrode force exerting a stronger effect on weld quality than weld time. All welds produced below 4000 amperes at 30 kgf failed through interfacial mode, while weld nuggets above 3 mm diameter within the lobe curve exhibited complete pull out failure. Hardness testing was conducted on samples welded at 40 kgf and revealed that variation in current had a greater influence on fusion zone hardness than changes in weld time from 2 to 5 cycles when current was held constant. Specifically, specimens produced at 2000 amperes showed the highest fusion zone hardness, those at 3000 amperes exhibited intermediate hardness, and samples at 4000 and 5000 amperes had hardness values in the welded zone that closely matched the parent metal. The fusion zone hardness in optimal welds increased to around 200 to 210 HV, while the HAZ in most samples exhibited recrystallization, resulting in hardness values slightly higher than the base metal 195 to 210 HV. In a few samples produced at lower welding current, hardness exceeded 220 HV. Microstructural analysis confirmed the presence of skeletal, lathy, and acicular delta ferrite in both the fusion and heat affected zones, depending on welding current, time, and force. Welds formed with higher current or sufficient heat input favored the development of skeletal delta ferrite, while rapid cooling due to lower heat input resulted in lathy and acicular delta ferrite. All welds exhibited columnar grain growth in the direction of the water cooled electrodes and a distinct heat affected zone surrounding the fusion zone.

Abstract: The purpose of the present work is studying feature of structure and phase structure thermal processed high-strength pig-iron with spherical graphite. Re­search has been dictated by difficulty of differentiation of structural compo­nents in the normalized pig-iron, es­pecially perlite and top of bainite. It is explained, difficulties of differentiation in nickel pig-iron perlite (sor­bite, trostite) and top of bainite: a plenty austenite at the given way of heat treat­ment undergoes disintegration on evtektoide to the mechanism, and separate plates of bainite ferrite are very fine, as defines similarity of structures at etching in nit­ric acid and their super­vi­sion at small increases. Samples of the same alloy (about 2%Ni), but cooled with the big speed indus­trially, get structure needle of bainite. It is confirmed, that the quantity austenite increases with rise in temperature austenition. It is installed, that in the pig-iron, cast in the metal form trans­formation occurs to education of smaller quantity residual austenite. This effect is connected with smaller micro liquation che­mi­cal elements, first of all silicon. It is established, that in the pig-iron, cast in the metal form by virtue of specific distribution of chemical elements at crystallization and crushing evtekti­ke grains, manganese is kept in the places corresponding sites ledeburite, and on borders of grains, silicon is distributed in regular more intervals, than in the pig-iron, cast in the sandy form. All this predetermines more uniform and full course of process γ–α of transformation.

Abstract: The diagrams of isothermal transformation based on kinetic curves R = (τ) for retained austenite in high-strength alloy low-carbon at overcooling have been built. It is shown that the temperature of quenching influences the stability at overcooling and resistance to isothermal transformation of austenite at sub-zero temperatures.

Abstract: It has been established that external uniaxial tensile stresses influence the character and kinetics of the g®a-transformation in cold-rolled martensite steel at sub-zero temperatures (down to -60 °C).

Abstract: We conducted comparative tests of the wear resistance of metals operating under abrasive conditions. Samples were cut from the working parts of mixer-pneumosuperchargers. The chemical composition and mechanical properties were determined. To compare samples under abrasive wear conditions, we designed and assembled a carousel installation. The principle of its operation is based on mixing the abrasive medium by the samples being studied with a given speed. Wear resistance was evaluated by weight loss by samples after several test cycles. To determine changes in the structure of the metal during abrasive wear, metallographic studies of the samples were carried out before and after the tests. It is shown that the best complex of service and mechanical properties is possessed by 110G13L steel.

Abstract: A high strength austenitic steel is expected as a structural material for cryogenic use because fcc material does not cause a cleavage fracture despite high strength. High manganese steel which is a strong candidate material of the cryogenic high strength austenitic steel was originally famous for the Hadfield steel and widely applicable in actual use. In general, an excellent cryogenic toughness of the high manganese steels is achieved by obtaining stable fcc microstructure with an adequate amount manganese which is a typical austenite former alloy. However, as addition of manganese is not effective for increasing strength, other strengthening alloying elements like carbon and chromium need to be added. In this study, an effect of alloying elements on strength and cryogenic toughness of the high manganese austenitic steel is studied.

Abstract: This study was presented due to the increasing demand of High Strength Low Alloy (HSLA) steel, such as demand for thinner-walled and large diameter pipes in oil and gas industries. In order to meet the imposed economic restrictions, the high standard of all kinds of steel properties is required and can be achieved by controlling the steel microstructure. The austenite grain size influences the microstructure and properties of steel significantly, in which fine austenite grain size leads to higher strength, better ductility, and higher toughness. Studying the behavior of steel grain growth during the reheating process is still being a fascinating subject. P.R. Rios and D Zollner [1] mentioned that grain growth is the most important unresolved issue that has been a topic of research for many years. In this research, the behavior of austenite grain growth at a high niobium-low carbon (High Nb-low C) and low Nb-high C HSLA steel was evaluated, and the result was compared with other investigation. The results found that the austenite grain growth at high Nb-high C steel was slower than the growth at a low Nb-low C steel. The activation energy of austenite grain growth and both constant A and exponent n ware determined close agreement was obtained between the prediction of the model and the experimental grain size value.

Abstract: The paper is devoted to an experimental investigation of a high-temperature deformation in V-alloyed high-nitrogen austenitic Fe-19Cr-22Mn-1.5V-0.3C-0.6N steel processed via different thermo-mechanical treatments. Simple thermo-mechanical processing regimes (cold rolling or rolling with single post-deformation anneal) do not allow to realize a substantial elongation in high-nitrogen steel during high-temperature tensile tests. For fine-grained austenitic structure with an average grain size of 3 µm, the maximal value of elongation to failure of 150% was realized at temperature 950 °C. Using a multi-stage thermo-mechanical treatment included cold rolling and intermediate anneals, a heterophase grain/subgrain structure with high density of deformation-induced defects and precipitates was produced. When heated to a deformation temperature, this deformation-assisted microstructure recrystallizes into a stable fine-grained structure and demonstrates the attributes of superplastic flow (values of elongation to failure higher than 400%) in the temperature range of 850-1000 °C. The maximum elongation of 900% is achieved at temperature of 950 °C and an initial strain rate of 10^-4 s^-1.

Abstract: In the present work, the segregation degrees of ferrite and austenite stabilizer alloying elements were analyzed for a high strength steel. For this, samples were taken from the surface and center of the hot-top and the upper section of a 40 MT ingot. The results showed that the positive segregation ratios for all the investigated elements were higher in the ingot center than in the surface with higher values for austenite stabilizer elements. The increase of austenite alloying element stabilizers was accompanied by the change in the primary solidification mode of the austenite phase. The obtained results are in good agreement with the observed presence of austenite, revealed by X-ray diffraction analysis, stabilized by the austenite alloying elements.

Abstract: An important factor in solving the problem of stainless steel corrosion resistance is carbon concentration reduction. However, a decrease in carbon content of austenitic steels leads to a drop in level of their strength properties. Theoretically, nitrogen alloying can lead to a strength increase in all types of austenitic corrosion-resistant steels. Practically, nitrogen alloying is effectively only with low-carbon compositions. This work shows the effect of nitrogen on the mechanical properties of middle-alloying nitrogen, containing stainless steel, and a study of AISI 304L and pilot steel with different nitrogen content (from 0.16 to 0.30 wt. %). Nitrogen increases strength of steel, which is approximately 30-60% higher than for steel without nitrogen, but reduces technological plasticity. Pilot steels show high corrosion resistance and fine austenite grains.